I. Classes of Data Communications
Data communication connections may be classified by the type of connection which is established between the caller and callee (or endpoint). The two types of connections are (1) leased, wherein the connecting line permanently connects the caller and endpoint; and (2) switched, wherein a connection may be established between a caller and any of one of multiple endpoints as required. Data communications connections may also be classified by the type of transmission technique which is used. They are (1) asynchronous (start/stop), wherein characters are transported as individual elements; and (2) synchronous, wherein characters are transported in contiguous packets. Thus, the data communications environment is divided into four classes represented by the combinations of the type of connection and transmission technique used.
The asynchronous leased-line class is virtually empty. With the possible exception of outmoded industrial or governmental installations persisting from the period when switched-line data communication was not believed to be feasible, there are few users in this class of Data Communications.
Formerly, the concentration was on synchronous leased-line concentrators and terminal controllers. These uses have become far outnumbered due to the proliferation of inexpensive personal computers and workstations which almost invariably use asynchronous transmission. These single user devices often include asynchronous communications capability as a standard feature of the machine.
Since switched-line communication is generally significantly less expensive than leased-line communication, and it is much more flexible, its acceptance has been much greater. Often no separate line cost need be considered. The popularity of switched-line communication is enhanced because it enables a personal computer to share an analog phone line otherwise used for voice communications. Thus, the most rapidly growing class of data communications is the asynchronous switched-line class.
II. Analog Technology in Data Communications
Despite the advent of digital networks, the use of analog technology to carry data has continuously increased. The number of modems which have been installed, especially asynchronous, switched-line modems, has grown significantly, and is expected to continue to do so in the future. This continued growth of analog technology in data communications has been attributed to a number of factors.
First, asynchronous modem capabilities have significantly improved since their introduction in the 1970s. Originally, data rates of asynchronous modems were from 0 to 300 bits per second (bps). Today, asynchronous modems have the capability of transferring data at rates of 14,400 bps and higher. For example, there are some proprietary modems with data rates of 28,800 bps. This represents a growth of nearly two orders of magnitude from their introduction.
Second, the upgrading of asynchronous modems enabled user systems to maintain downward application compatibility. Downward application compatibility is the capability to upgrade modem technology without requiring the addition of devices or software for the specific purpose of achieving workable interface connections between the applications programs and the modem. Downward application compatibility is maintained due to the continued use of a standard communications interface and the development of a de facto standard software interface.
Although modem technology has evolved through five generations in ten years, the interface to the modem has remained a constant: The EIA Standard RS-232, established by the Electronic Industries Association or its European equivalent, CCITT recommendations V.24 and V.28, established by the International Consultative Committee on Telegraphy and Telephony has been used throughout the evolution.
The original IBM Personal Computer lacked the ability to completely utilize the capabilities of the serial communications hardware. To utilize these capabilities with the limited power available, applications adopted the practice of directly manipulating the hardware. Thus, the image of a National Semiconductor INS 8250 Universal Asynchronous Receiver/Transmitter (UART), became a new standard interface between the applications programs and the communications hardware. Extensions have also been made to the UART, but these extensions maintain compatibility with older versions.
When accessed directly, the UART may be set to communication speeds from less than 300 bps to beyond 115 Kbps. Every time faster modems became available, the user merely changed a command or configuration to upgrade. Because the UART is accessed in the same manner for all communication rates, downward application compatibility has been maintained as data speeds were increased.
In addition to having a standard communications interface available, applications were able to take advantage of a de facto standard interface for setting and querying modem parameters and status, and for call setup and takedown. The AT command set, originally created by Hayes Corporation, was designed for use with non-programmable terminals. Its purpose was to permit control of the modem from the terminal by use of in-band signals. While this command set has been extended several times, the extensions have been consistent, and the base set has enabled applications to pass commands across the hardware interface. Thus, downward application compatibility was achieved. Expensive application software did not have to be replaced, and custom applications did not have to be rewritten as each new generation of modems was introduced.
Third, the upgrading of asynchronous modems enabled user systems to maintain downward connectivity compatibility. Similar to downward applications compatibility, downward connectivity compatibility is the capability to upgrade modem technology without the addition of devices or software for the specific purpose of achieving workable interface connections between the upgrade modems and the endpoints with which it communicates. Downward connectivity compatibility was achieved by requiring the faster speed modems to communicate at all the previous lower speeds and to maintain a standard communication protocol.
III. Introduction of ISDN
The Integrated Services Digital Network (ISDN) represents an emerging technology which is aimed at replacing the existing analog telephone network with an all digital network capable of handling digital communications as well as voice communications. Numerous suppliers are now offering ISDN interface adapters for personal computers; yet, sales of these products have been disappointingly low. In spite of the wealth of benefits to be achieved by installing ISDN, it has not yet received wide acceptance.
Several interrelated reasons have been identified for preventing the growth of ISDN. These include the high cost of ISDN equipment due to low sales volumes, the high cost and unavailability of ISDN applications, and the unavailability of support services.
Another reason given is that ISDN deployment is expected to be fragmented. Metropolitan areas may be served before rural areas; those areas with relatively strong infrastructures before the weak. This implementation of ISDN "islands" has inhibited ISDN acceptance because users are unwilling to replace a service that has no limitation on calling area with a service that can only reach nearby endpoints.
However, among the various problems impeding the growth of ISDN, connectivity seems to be the most significant factor. Connectivity refers to the ability of data communications equipment to operate and interface with both, the ISDN (digital) network and the analog network. This limitation effectively limits the set of ISDN potential ISDN users to those who create new networks of users that connect to only new endpoints, thereby enabling them to be completely digital.
For example, a user that is presently connected to greater than one endpoint who finds his productivity inhibited by bottlenecked data flow to or from a connection-point may be tempted to switch to ISDN as soon as the other endpoint did so. He would be less enthusiastic when he realized that he would have to maintain an analog telephone line and analog modem and adapter as well in order to continue to maintain communications with his other endpoints.
Another example of where upgrading present analog systems is more advantageous than converting to ISDN is when a number of users are in a wide-spread existing network. It is essentially impossible to upgrade all the endpoints in a network at once. However, in an analog network, piecemeal upgrading is possible because of the downward compatibility characteristics discussed above. It is possible, for example, for a user to get a 9600 bps modem to be used at 2400 bps in anticipation of a future network upgrade to 9600 bps.
One conventional ISDN adapter is described in commonly owned U.S. Pat. No. 4,991,169 to Davis et al. This adapter utilizes an ISDN Primary Rate adapter card with 30 time slots (time-division multiplexing of a 2.048 MHz serial bit stream), and allowed each time slot to be connected to a different analog modem at the other end of the network. This implementation focuses on connecting a computer to a multi-channel environment wherein many lines are terminated for a data base, terminal, or other remote processor access. This system, however, does not provide the capability to connect a computer to digital and analog devices via a single channel in a data link. In addition, the adapter disclosed in the above patent only communicates over channels in the time division multiplex link that are log PCM encoded for voice band analog signals.
Another conventional solution has been to communicate with both analog and digital devices using multiple interfaces; one for digital and one for analog. To implement such a solution, a system may have an adapter card that has a modem driving the analog interface and digital hardware driving the digital interface. This dual interface function does not have the benefits of a single interface nor does it have the benefits of communicating in an all digital method.
Another conventional solution is to use digital to analog and analog to digital convertors to convert modem signal to digital form prior to transmitting them over the digital network. One implementation of this approach has been to connect an external modem to an analog port in an ISDN adapter. The ISDN adapter digitizes the modem signals, multiplexes the signals with other digital signals, and then transmits them on to the digital network. The output of the remote modem is digitized by the network and is multiplexed into a digital channel to a receiver which demultiplexes the digital signal into several digital channels in the receiver which then converts the digital signal back to analog. However, the approach of adding an analog to digital converter to a modem chip results in distortion due to the multiple conversions which occur. In addition to distortion, these solutions also have a loss of accuracy with each conversion. This technique, however, has a higher cost than other techniques due to the extra components used for the analog conversions.
Since ISDN is a switched service, and since the greatest growth is in the asynchronous switched-line data communications class, it is desired to provide a means for ISDN users to interface with the asynchronous switched-line analog networks. Given the problems associated with the growth of ISDN discussed above, the adapter must be application downward compatible and downward connectivity compatible.